nervous tissue
DESCRIPTION
Nervous Tissue. Controls and integrates all body activities within limits that maintain life Three basic functions sensing changes with sensory receptors fullness of stomach or sun on your face interpreting and remembering those changes reacting to those changes with effectors - PowerPoint PPT PresentationTRANSCRIPT
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Nervous Tissue
• Controls and integrates all body activities within limits that maintain life
• Three basic functions– sensing changes with sensory receptors
• fullness of stomach or sun on your face
– interpreting and remembering those changes
– reacting to those changes with effectors• muscular contractions• glandular secretions
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Major Structures of the Nervous System
• Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors
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Organization of the Nervous System
• CNS is brain and spinal cord• PNS is everything else
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Nervous System Divisions
• Central nervous system (CNS) – consists of the brain and spinal cord
• Peripheral nervous system (PNS)– consists of cranial and spinal nerves that contain
both sensory and motor fibers– connects CNS to muscles, glands & all sensory
receptors
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Subdivisions of the PNS
• Somatic (voluntary) nervous system (SNS)– neurons from cutaneous and special sensory
receptors to the CNS– motor neurons to skeletal muscle tissue
• Autonomic (involuntary) nervous systems– sensory neurons from visceral organs to CNS– motor neurons to smooth & cardiac muscle and glands
• Sympathetic division (speeds up heart rate)• Flight/Fight ( Thoracolumbar output T1-L2 )
• Parasympathetic division (slow down heart rate)• CranioSacral output C.N III, VII, IX, X, S2-S4
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Neurons• Functional unit of nervous system• Have capacity to produce action potentials
– electrical excitability• Cell body
– single nucleus with prominent nucleolus– Nissl bodies (chromatophilic substance)
• rough ER & free ribosomes for protein synthesis
– neurofilaments give cell shape and support– microtubules move material inside cell– lipofuscin pigment clumps (harmless aging)
• Cell processes = dendrites & axons
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Nucleus with Nucleolus
Parts of a Neuron
Axons or Dendrites
Cell body
Neuroglial cells
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Dendrites• Conducts impulses
towards the cell body
• Typically short, highly branched & unmyelinated
• Surfaces specialized for contact with other neurons
• Contains neurofibrils & Nissl bodies
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Axons• Conduct impulses away
from cell body• Long, thin cylindrical process
of cell • Arises at axon hillock • Impulses arise from initial
segment (trigger zone)• Side branches (collaterals)
end in fine processes called axon terminals
• Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters
• ACHSynaptic boutons
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Axonal Transport• Cell body is location for most protein synthesis
– neurotransmitters & repair proteins
• Axonal transport system moves substances – slow axonal flow
• movement in one direction only -- away from cell body• movement at 1-5 mm per day
– fast axonal flow• moves organelles & materials along surface of microtubules• at 200-400 mm per day• transports in either direction• for use or for recycling in cell body
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Axonal Transport & Disease• Fast axonal transport route by which
toxins or pathogens reach neuron cell bodies– tetanus (Clostridium tetani bacteria)– disrupts motor neurons causing painful
muscle spasms
• Bacteria enter the body through a laceration or puncture injury– more serious if wound is in head or neck
because of shorter transit time
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Functional Classification of Neurons• Sensory (afferent) neurons
– transport sensory information from skin, muscles, joints, sense organs & viscera to CNS
• Motor (efferent) neurons– send motor nerve impulses to muscles &
glands
• Interneurons (association) neurons– connect sensory to motor neurons– 90% of neurons in the body
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Structural Classification of Neurons
• Based on number of processes found on cell body– multipolar = several dendrites & one axon
• most common cell type– bipolar neurons = one main dendrite & one axon
• found in retina, inner ear & olfactory – unipolar neurons = one process only(develops from a bipolar)
• are always sensory neurons ( Dorsal Root Ganglion )
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Association or Interneurons• Named for histologist that first described
them or their appearance
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• Half of the volume of the CNS • Smaller cells than neurons• 50X more numerous• Cells can divide
– rapid mitosis in tumor formation (gliomas)
• 4 cell types in CNS– astrocytes, oligodendrocytes, microglia & ependymal
• 2 cell types in PNS– schwann and satellite cells
Neuroglial Cells
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Astrocytes• Star-shaped cells• Form blood-brain
barrier by covering blood capillaries
• Metabolize neurotransmitters
• Regulate K+ balance
• Provide structural support
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Oligodendrocytes
• Most common glial cell type
• Each forms myelin sheath around more than one axons in CNS
• Analogous to Schwann cells of PNS
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Microglia
• Small cells found near blood vessels• Phagocytic role -- clear away dead cells• Derived from cells that also gave rise to
macrophages & monocytes
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Ependymal cells
• Form epithelial membrane lining cerebral cavities & central canal
• Produce cerebrospinal fluid (CSF)
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Satellite Cells
• Flat cells surrounding neuronal cell bodies in peripheral ganglia
• Support neurons in the PNS ganglia
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Schwann Cell
• Cells encircling PNS axons• Each cell produces part of the myelin sheath
surrounding an axon in the PNS
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Myelination in PNS
• Schwann cells myelinate (wrap around) axons in the PNS during fetal development
• Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath
• Tube guides growing axons that are repairing themselves
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Myelination in the CNS• Oligodendrocytes myelinate axons in the CNS • Broad, flat cell processes wrap about CNS axons, but the
cell bodies do not surround the axons• No neurilemma is formed• Little regrowth after injury is possible due to the lack of
a distinct tube or neurilemma
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Gray and White Matter
• White matter = myelinated processes (white in color)• Gray matter = nerve cell bodies, dendrites, axon terminals,
bundles of unmyelinated axons and neuroglia (gray color)– In the spinal cord = gray matter forms an H-shaped inner core
surrounded by white matter– In the brain = a thin outer shell of gray matter covers the surface & is
found in clusters called nuclei inside the CNS
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Lecture 2 Electrical Signals in Neurons
• Neurons are electrically excitable due to the voltage difference across their membrane
• Communicate with 2 types of electric signals– action potentials that can travel long
distances– graded potentials that are local membrane
changes only
• In living cells, a flow of ions occurs through ion channels in the cell membrane
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Two Types of Ion Channels• Leakage (nongated) channels are always open
– nerve cells have more K+ than Na+ leakage channels
– as a result, membrane permeability to K+ is higher– explains resting membrane potential of -70mV in
nerve tissue• Gated channels open and close in response to a
stimulus results in neuron excitability– voltage-gated open in response to change in
voltage– ligand-gated open & close in response to particular
chemical stimuli (hormone, neurotransmitter, ion)– mechanically-gated open with mechanical
stimulation
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Gated Ion Channels
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Resting Membrane Potential
• Negative ions along inside of cell membrane & positive ions along outside– potential energy difference at rest is -70 mV – cell is “polarized”
• Resting potential exists because– concentration of ions different inside & outside
• extracellular fluid rich in Na+ and Cl-• cytosol full of K+, organic phosphate & amino acids
– membrane permeability differs for Na+ and K+• 50-100 greater permeability for K+• inward flow of Na+ can’t keep up with outward flow of K+• Na+/K+ pump removes Na+ as fast as it leaks in
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Lecture 2 Graded Potentials
• Small deviations from resting potential of -70mV– hyperpolarization = membrane has become more
negative– depolarization = membrane has become more positive
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How do Graded Potentials Arise?• Source of stimuli
– mechanical stimulation of membranes with mechanical gated ion channels (pressure)
– chemical stimulation of membranes with ligand gated ion channels (neurotransmitter)
• Graded/postsynaptic/receptor or generator potential– ions flow through ion channels and change
membrane potential locally– amount of change varies with strength of stimuli
• Flow of current (ions) is local change only
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Action Potential
• Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state
• Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization)
• All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second)
• Travels (spreads) over surface of cell without dying out
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Depolarizing Phase of Action Potential
• Chemical or mechanical stimuluscaused a graded potential to reachat least (-55mV or threshold)
• Voltage-gated Na+ channels open& Na+ rushes into cell– in resting membrane, inactivation gate of sodium channel is open
& activation gate is closed (Na+ can not get in)– when threshold (-55mV) is reached, both open & Na+ enters– inactivation gate closes again in few ten-thousandths of second– only a total of 20,000 Na+ actually enter the cell, but they change
the membrane potential considerably(up to +30mV)• Positive feedback process
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Repolarizing Phase of Action Potential
• When threshold potential of-55mV is reached, voltage-gated K+ channels open
• K+ channel opening is muchslower than Na+ channelopening which caused depolarization
• When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops)
• K+ outflow returns membrane potential to -70mV• If enough K+ leaves the cell, it will reach a -90mV membrane
potential and enter the after-hyperpolarizing phase• K+ channels close and the membrane potential returns to the resting potential of -
70mV
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Refractory Period of Action Potential
• Period of time during whichneuron can not generateanother action potential
• Absolute refractory period– even very strong stimulus will
not begin another AP– inactivated Na+ channels must return to the – resting state before they can be reopened
– large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible
• Relative refractory period– a suprathreshold stimulus will be able to start an AP– K+ channels are still open, but Na+ channels have closed
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The Action Potential: Summarized• Resting membrane potential is -70mV• Depolarization is the change from -70mV to +30 mV• Repolarization is the reversal from +30 mV back to -70 mV)
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Propagation of Action Potential
• An action potential spreads (propagates) over the surface of the axon membrane– as Na+ flows into the cell during
depolarization, the voltage of adjacent areas is effected and their voltage-gated Na+ channels open
– self-propagating along the membrane
• The traveling action potential is called a nerve impulse
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Local Anesthetics
• Prevent opening of voltage-gated Na+ channels
• Nerve impulses cannot pass the anesthetized region
• Novocaine and lidocaine
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Continuous VS Saltatory Conduction• Continuous conduction (unmyelinated fibers)
– step-by-step depolarization of each portion of the length of the axolemma
• Saltatory conduction– depolarization only at nodes of Ranvier where
there is a high density of voltage-gated ion channels
– current carried by ions flows through extracellular fluid from node to node
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Saltatory Conduction
• Nerve impulse conduction in which the impulse jumps from node to node
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Speed of Impulse Propagation• The propagation speed of a nerve impulse is not
related to stimulus strength. – larger, myelinated fibers conduct impulses faster due to size
& saltatory conduction
• Fiber types– A fibers largest (5-20 microns & 130 m/sec)
• myelinated somatic sensory & motor to skeletal muscle
– B fibers medium (2-3 microns & 15 m/sec)
• myelinated visceral sensory & autonomic preganglionic
– C fibers smallest (.5-1.5 microns & 2 m/sec)
• unmyelinated sensory & autonomic motor
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Action Potentials in Nerve and Muscle• Entire muscle cell membrane versus only the
axon of the neuron is involved
• Resting membrane potential– nerve is -70mV– skeletal & cardiac muscle is closer to -90mV
• Duration– nerve impulse is 1/2 to 2 msec– muscle action potential lasts 1-5 msec for skeletal &
10-300msec for cardiac & smooth
• Fastest nerve conduction velocity is 18 times faster than velocity over skeletal muscle fiber
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Lecture 3 Signal Transmission at Synapses
• 2 Types of synapses– electrical
• ionic current spreads to next cell through gap junctions• faster, two-way transmission & capable of
synchronizing groups of neurons
– chemical• one-way information transfer from a presynaptic
neuron to a postsynaptic neuron– axodendritic -- from axon to dendrite– axosomatic -- from axon to cell body– axoaxonic -- from axon to axon
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Chemical Synapses• Action potential reaches end
bulb and voltage-gated Ca+ 2 channels open
• Ca+2 flows inward triggering release of neurotransmitter
• Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors – the more neurotransmitter
released the greater the change in potential of the postsynaptic cell
• Synaptic delay is 0.5 msec• One-way information transfer
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Excitatory & Inhibitory Potentials• The effect of a neurotransmitter can be either
excitatory or inhibitory– a depolarizing postsynaptic potential is called
an EPSP• it results from the opening of ligand-gated Na+ channels• the postsynaptic cell is more likely to reach threshold
– an inhibitory postsynaptic potential is called an IPSP
• it results from the opening of ligand-gated Cl- or K+ channels
• it causes the postsynaptic cell to become more negative or hyperpolarized
• the postsynaptic cell is less likely to reach threshold
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Removal of Neurotransmitter• Diffusion
– move down concentration gradient
• Enzymatic degradation– acetylcholinesterase
• Uptake by neurons or glia cells– neurotransmitter
transporters– Prozac = serotonin
reuptake inhibitor
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• Summation of effects of neurotransmitters released from several end bulbs onto one neuron
Spatial Summation
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Temporal Summation
• Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron
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Neurotransmitter Effects
• Neurotransmitter effects can be modified– synthesis can be stimulated or inhibited– release can be blocked or enhanced– removal can be stimulated or blocked – receptor site can be blocked or activated
• Agonist– anything that enhances a transmitters effects
• Antagonist– anything that blocks the action of a
neurotranmitter
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Small-Molecule Neurotransmitters• Acetylcholine (ACh)
– released by many PNS neurons & some CNS– excitatory on NMJ but inhibitory at others– inactivated by acetylcholinesterase
• Amino Acids– glutamate released by nearly all excitatory
neurons in the brain ---- inactivated by glutamate specific transporters
– GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)
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• Biogenic Amines– modified amino acids (tryptophan)
• norepinephrine -- regulates mood, dreaming, awakening from deep sleep
• dopamine -- regulating skeletal muscle tone• serotonin -- control of mood, temperature
regulation, & induction of sleep
Small-Molecule Neurotransmitters (2)
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• ATP and other purines (ADP, AMP & adenosine)– excitatory in both CNS & PNS– released with other neurotransmitters (ACh & NE)
• Gases (nitric oxide or NO)– formed from amino acid arginine by an enzyme– formed on demand and acts immediately
• diffuses out of cell that produced it to affect neighboring cells
• may play a role in memory & learning
– first recognized as vasodilator that helps lower blood pressure
Small-Molecule Neurotransmitters (3)
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Neuropeptides• 3-40 amino acids linked by peptide bonds
• Substance P -- enhances our perception of pain
• Pain relief– enkephalins -- pain-relieving effect by
blocking the release of substance P – acupuncture may produce loss of pain
sensation because of release of opioids-like substances such as endorphins or dynorphins
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Regeneration & Repair
• Plasticity maintained throughout life– sprouting of new dendrites– synthesis of new proteins– changes in synaptic contacts with other
neurons
• Limited ability for regeneration (repair)– PNS can repair damaged dendrites or
axons– CNS no repairs are possible
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Neurogenesis in the CNS
• Formation of new neurons from stem cells was not thought to occur in humans– 1992 a growth factor was found that stimulates adult
mice brain cells to multiply– 1998 new neurons found to form within adult human
hippocampus (area important for learning)
• Factors preventing neurogenesis in CNS– inhibition by neuroglial cells, absence of growth
stimulating factors, lack of neurolemmas, and rapid formation of scar tissue
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Multiple Sclerosis (MS)• Autoimmune disorder causing
destruction of myelin sheaths in CNS– sheaths becomes scars or plaques– 1/2 million people in the United States– appears between ages 20 and 40 – females twice as often as males
• Symptoms include muscular weakness, abnormal sensations or double vision
• Remissions & relapses result in progressive, cumulative loss of function
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• The second most common neurological disorder – affects 1% of population
• Characterized by short, recurrent attacks initiated by electrical discharges in the brain– lights, noise, or smells may be sensed– skeletal muscles may contract involuntarily– loss of consciousness
• Epilepsy has many causes, including;– brain damage at birth, metabolic disturbances,
infections, toxins, vascular disturbances, head injuries, and tumors
Epilepsy
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Neuronal Structure & Function